I officially have never played any of them, which I feel is a tragedy. Honestly I want to play Legacy of Kain 2 first, seems the most my style, but I worry the story won't make sense since there were 3 games before that.
Here's a quick rundown:
Blood Omen: sets up a lot of the world-building and backstory, so worth playing if you can find it, get it running, and don't mind playing such an old action/RPG. I still found it to be a solid game. That said, if you don't want the hassle you can always just read a plot summary, and it's not impossible
to muddle your way through the plot of the later games without it (though it helps a lot).
Soul Reaver: Probably the lightest on 'plot' of any game in the series; you really only need to see the intro, ending, and maybe one or two cutscenes from the middle to have a good sense of what happens. (Most of the game is just "Raziel hunts down his brothers and kills them.") IIRC, Soul Reaver 2 includes the intro from SR1 as a bonus, and the intro to SR2 is basically a graphically updated retelling of the ending of SR1, so it's actually fairly safe to skip SR1.
Soul Reaver 2: Pound for pound, this game has the most story of any entry in the series, and it's also my favourite. Not to be missed.
Blood Omen 2: Ugh. This was handled by a different development team who completely botched the storyline. (Characters are alive who are supposed to be dead, with no explanation, etc.) It's also just... not a good game, from a story or gameplay perspective. Can be safely skipped (and I heartily recommend doing so), since the next game pretty much pretends this one never happened.
Defiance: Spends a loooooooong time recapping the backstory from the previous games, which is pretty boring if you're already familiar with it, but when the game finally gets rolling in the third act, it does an awesome wrap-up of the series' storyline.
So to summarize: BO1 recommended but optional, SR1 optional, SR2 essential, BO2 so awful it's basically non-canon, Defiance worth it if you've stuck around this long.